The present invention relates to a small air-cooled two-stroke internal combustion engine having a displacement in the range of from about 15 cc to about 65 cc, which is suited for use in, for example, a portable small working machine such as a bush cutter or a chain saw, and in particular to a small air-cooled two-stroke internal combustion engine which is capable of minimizing hazardous components in the exhaust gas without badly affecting the output characteristics of the engine.
In view of the recent increasing concerns about environmental problems, a reduction in the amounts of hazardous components, such as HC, CO, NOx, PM (fine particles of unburnt components of oil), etc., in the exhaust gas discharged from an engine is now strongly demanded for even a small air-cooled two-stroke gasoline internal combustion engine to be used in a portable small working machine such as a bush cutter or a chain saw. For example, according to the exhaust gas control bill in California, known as CARB Tier II, it is required that, in the case of an SI (spark ignition) two-stroke internal combustion engine having a displacement of not more than 65 cc, HC+NOx should be reduced to 54 g/bhp-h or less, CO to 400 g/bhp-h or less, and PM to 1.5 g/bhp-h or less, beginning with the year 2000.
With a view to meeting such an exhaust gas regulation, the owner of the present invention have made various proposals concerning the structure of two-stroke internal combustion engines. One of them, as disclosed in Japanese Patent Unexamined Publication 9-280057, is a modification of the timing of the opening and closing of an exhaust port and a scavenging port of the cylinder by the piston in a two-stroke internal combustion engine having a Schnurle type scavenging system. In particular, the opening and closing timing is shortened in terms of crank angle as compared with that of a conventional engine, the opening and closing timing of the exhaust port by the piston being set to an angle in the range of from 100 to 120 degrees in terms of the crank angle with respect to the bottom dead center disposed therebetween, while the opening and closing timing of the scavenging port by the piston is set to an angle in the range of from 85 to 100 degrees in terms of crank angle with respect to the bottom dead center disposed therebetween.
The setting of the opening and closing timing of the exhaust port as well as of the scavenging port of the cylinder are achieved by lowering the position of the upper edge of the exhaust port as well as the position of the upper edge of the scavenging port, and at the same time, by shortening the distance between the upper edge of the exhaust port and the upper edge of the scavenging port.
According to the conventional engine of the same kind as mentioned above, the output characteristics of the engine are mainly taken into account in general, so that the opening and closing timing of the exhaust port is generally set to an angle in the range of 130 to 150 degrees, while the opening and closing timing of the scavenging port is generally set to an angle in the range of 100 to 110 degrees. However, since the opening and closing timings of these ports are to be set as mentioned above in the case of Japanese Patent Unexamined Publication H9-280057, the exhaust port and the scavenging port are opened at a delayed timing as compared with that of the conventional engine in the descending stroke, while the exhaust port and the scavenging port are closed at an advanced timing as compared with that of the conventional engine in the ascending stroke.
Therefore, the energy of combustion is sufficiently converted into a thrusting force to push the piston downward before reaching the point at which exhaust is initiated, i.e., the time when the exhaust port starts to open, thus minimizing the exhaust pressure. As a result, the scavenging air flow is prevented from being pushed back, thus making it possible to increase the flow rate of the scavenging gas flow and to effectively perform the scavenging.
Since the scavenging is performed effectively, the quantity of the blow-by of fresh gas (air-fuel mixture) through the exhaust port is minimized. As a result, the quantity of HC components in the exhaust gas are minimized and at the same time the output of the engine is improved. Furthermore, since the modification of the opening and closing timing are effected by simply changing the shapes and positions of the exhaust port and of the scavenging port, the modification does not result in an increase in the manufacturing cost of the engine.
In the ordinary two-stroke internal combustion engine having a Schnurle type scavenging system, as shown in the aforementioned Japanese Patent Unexamined Publication H9-280057 (see FIG. 3), a pair of scavenging ports 9 and 9 are formed symmetrically with respect to a longitudinal sectional plane (F) that bisects an exhaust port 10 (a so-called double flow scavenging system), thereby permitting part of the scavenging flow of the air-fuel mixture that has been injected from or blown out of the pair of scavenging ports 9 and 9 to impinge against an inner wall (cylinder bore) of the stationary cylinder. Additionally, there is also known a so-called four-flow scavenging system comprising two pairs of scavenging ports.
FIG. 8 shows one example of a conventional two-stroke internal combustion engine having a four-flow scavenging system, wherein the cylinder 2' shown therein is provided, on an exhaust port 10' side thereof, with a pair of first scavenging ports 9A', 9A' which are disposed symmetrically with respect to a longitudinal sectional plane F' that bisects the exhaust port 10' and, on the other side opposite to the exhaust port 10' thereof, with a pair of second scavenging ports 9B', 9B' which are disposed symmetrically with respect to the longitudinal sectional plane F'.
The inner horizontal scavenging angle .alpha.' formed close to the exhaust port 10' and the outer horizontal scavenging angle .beta.' formed remote from the exhaust port 10' by a pair of scavenging flows blown out of the pair of first scavenging ports 9A', 9A' are set to about 100 degrees (for example, 94 degrees) and about 120 degrees, respectively, while the inner horizontal scavenging angle .gamma.' formed close to the exhaust port 10' and the outer horizontal scavenging angle .delta.' formed remote from the exhaust port 10' by a pair of scavenging flows blown out of the pair of second scavenging ports 9B', 9B' are set to about 120 degrees and about 150 degrees, respectively. Additionally, the horizontal cross-sectional area Sa' of each of the first scavenging ports 9A', 9A' is made smaller than the horizontal cross-sectional area Sb' of each of the second scavenging ports 9B', 9B' (Sa'&lt;Sb').
In the conventional two-stroke internal combustion engine with a four-flow scavenging system, a pair of scavenging flows blown out of the pair of first scavenging ports 9A', 9A' are caused to impinge against each other, and at the same time, a pair of scavenging flows blown out of the pair of second scavenging ports 9B', 9B' are also caused to impinge against each other, so that the relative impinging velocity between these scavenging flows is doubled and the energy produced by the impingement is quadrupled as compared with the aforementioned engine having a double flow scavenging system where part of the scavenging flow of the air-fuel mixture impinges against the inner wall (cylinder bore) of the stationary cylinder.
As a result, the atomization of fuel particles in the air-fuel mixture that are not sufficiently atomized before they are blown out of each scavenging port is greatly promoted, whereby the ignitability and combustion efficiency of the air-fuel mixture are greatly improved. As a result, the exhaust gas clarification capability as well as fuel consumption of the engine are improved.
However, it is still impossible with the two-stroke internal combustion engine described above to prevent part of the fresh gas (air-fuel mixture), even if a very small amount, from directly passing through the exhaust port. Even if the quantity of so-called blow-by of fresh gas is reduced to a certain extent, the improvement obtainable is not large enough, and hence the reduction of hazardous components (in particular, HC components) in the exhaust gas discharged from the engine is not necessarily satisfactory. Furthermore, if it is desired to further reduce the quantity of blow-by according to the aforementioned prior art, it would inevitably lead to a significant deterioration of the output of the engine.
Even in the case of the aforementioned two-stroke internal combustion engine with a four-flow scavenging system, the effect of promoting the atomization of fuel particles in the air-gas mixture is not sufficient, thus leaving room for further improvement.